Assessment of selected constituents in surface water of the upper Snake River basin, Idaho and western Wyoming, water years 1975-89

In 1991, a water-quality investigation of
the upper Snake River Basin was initiated as part
of the U.S. Geological Survey's National Water-Quality Assessment Program. The initial task of
the assessment was to compile and analyze available nutrient, suspended sediment, and pesticide
data collected in the basin. For analysis of nutrients
and suspended sediment, data collected during
water years 1980-89 were used. For pesticides, an
additional 5 years of data were included for a total
assessment period encompassing water years
1975-89.
Nearly 9,000 analyses of nutrients and suspended sediment from more than 450 stations were
retrieved from the U.S. Environmental Protection
Agency STORET and U.S. Geological Survey
WATSTORE data bases. Nineteen stations had
sufficient analyses for quantitative assessment. Of
the 19 stations analyzed, 4 are located on relatively
unaffected stream reaches, 8 are at or near mouths
of tributary basins affected by agricultural activities, and 7 are on the main stem of the Snake River.
Data indicate that nitrite plus nitrate and total
phosphorus concentrations generally increased in a
downstream direction along the Snake River; concentrations were largest at the mouths of drainage
basins tributary to the Snake River. Water-quality
stations were categorized as unaffected or minimally affected, agriculturally affected, or main
stem to compare nutrient concentrations between
drainage basins of differing land use/land cover.
Concentrations of nitrite plus nitrate, total nitrogen,
dissolved orthophosphate, and total phosphorus
were significantly (p<0.05) larger at agriculturally
affected and main-stem stations than at unaffected
stations; and concentrations of nitrite plus nitrate,
total nitrogen, and total phosphorus at agriculturally affected stations were significantly larger than
at main-stem stations. Significant differences in
seasonal concentrations of some nutrient species
also were noted.
Few suspended sediment and pesticide data
were available for the study basin. Only six stations
had sufficient data for quantitative assessment of
suspended sediment. A direct positive relation
exists between suspended sediment concentration
and streamflow; concentrations are largest in April,
May, and June at high streamflow. Most of the
pesticide data compiled from STORET and
WATSTORE were collected during water years
1975-79. Only 33 pesticide samples, excluding
samples collected for a Rural Clean Water
Program, were collected from surface water and
bottom sediment during water years 1980-89.
Bottom sediment collected near the mouth of the
Henrys Fork during the late 1970's had the largest
concentrations of pesticides in the basin; DOT,
ODD, and DDE concentrations exceeded 10 micrograms per kilogram.
Mass movement of nutrients and suspended
sediment in the upper Snake River Basin is
controlled primarily by changes in streamflow.
Between two and three times as much total nitrogen, total phosphorus, and suspended sediment
were transported out of the basin in water year
1984 (high-flow year) compared with 1989 (low-flow year). Reservoirs on the main stem of the
Snake River probably trap much of the nutrient and
most of the suspended sediment load generated
from upper parts of the basin.
A more extensive data-collection program in
the upper Snake River Basin is needed to address a
number of water-quality issues. These include
an analysis of effects of land use on the quality of
surface water; quantification of mass movement of
nutrients and suspended sediment at key locations
in the basin; distribution of aquatic organisms; and
temporal and spatial distribution of pesticides in
surface water, bottom sediment, and biota.